Process for producing a protective film on an aluminum surface

ABSTRACT

A process is disclosed for producing a firmly adhered, corrosion resistant film on an aluminum or aluminum alloy surface by means of anodization using an electrolyte containing at least one of the compounds selected from the group consisting of, aliphatic primary amines and aliphatic secondary amines which compounds act upon a coating to cause the same to adhere intimately to the aluminum surface on one hand and impart corrosion resistance to the latter on the other. The formation of the aluminum oxide layer and resinous coating may be carried out in one step or in two steps.

United States Patent [191 Ikegaya et al.

[4 1 Sept. 30, 1975 PROCESS FOR PRODUCING A PROTECTIVE FILM ON AN ALUMINUM SURFACE [73] Assignee: Riken Light Metal Industry Company Limited, Shizuoka, Japan 221 Filed: Oct. 24, 1972 2] Appl. No.: 300,159

[30] Foreign Application Priority Data v Oct. 22, I971 Japan 46-83340 [52] U.S. Cl. 204/38 A; 204/38 E [5]] Int. Cl. C25D 11/18 [58] Field of Search 204/58, 38 AE, 33, 181

[56] References Cited UNITED STATES PATENTS 2,855,35l 10/1958 Ernst 204/58 3,455,805 7/1969 Smith et al 204/l8l 3,632,486 l/I972 Herrmann et al.. 204/58 3,68l,224 8/1972 Stromberg 204/l8l 4/1973 Herrmann et al 204/58 ll/l973 Ikeda et al. 204/38 E OTHER PUBLICATIONS Metal Finishing Guidebook Directory, 36th Ed. 1968, Metals & Plastics Publications Inc., p. 536.

Modern Electroplating by F. A. Lowenheim, 2nd Ed., 1963, p. 632.

Primary ExaminerR. L. Andrews Attorney, Agent, or FirmBucknam and Archer 57 ABSTRACT A process is disclosed for producing a firmly adhered, corrosion resistant film on an aluminum or aluminum alloy surface by means of anodization using an electrolyte containing at least one of the compounds selected from the group consisting of, aliphatic primary amines and aliphatic secondary amines which compounds act upon a coating to cause the same to adhere intimately to the aluminum surface on one hand and impart corrosion resistance to the latter on the other. The formation of the aluminum oxide layer and resinous coating may be carried out in one step or in two steps.

1 Claim, No Drawings PROCESS FOR PRODUCING A PROTECTIVE FILM ON AN ALUMINUM SURFACE This invention relates to a process of producing on an aluminum or aluminum alloy surface a protective, ornamental and corrosion resistant film. More specifically, the present invention is concerned with the process in which an aluminum substrate is coated by immersion deposit with a continuous, smooth and firmly adhered resinous film. Yet more specifically, the invention is further concerned with the aluminum surface treatment in which an aluminum surface is provided with an anodized under-coat and a resinous over-coat simultaneously.

A number of processes have been hitherto been proposed for treating and finishing aluminum-bearing substrates so that product aluminum articles are resistant to environmental attack, i.e. corrosion by acidic or alkaline atmosphere and will present a continuous, smooth, level surface appearance. Prior to the application of a protective coating and/or an ornamental dye, the aluminum surface is subjected to an undercoat treatment such as by anodization and chemical conversion which has important bearing upon the properties of a protective film formed over the under-coat. Therefore, a variety of processes have also been proposed for forming an under-coat on an aluminum substrate. For example, there is known a process wherein a film is formed by chemical conversion on an aluminum surface, followed by application of a protective outer coating. According to this process, the protective resinous film is securely adhered to the under-coated film but the coated aluminum surface is not satisfactorily corrosionresistant. There is also known a process wherein the aluminum surface is anodized prior to the application of a protective coating. The anodized surface available by this process is well resistant to corrosion and superior in many respects chemically converted aluminum surface. But, the anodization employed therefor is extremely costly. In order to overcome the above drawbacks, there has been proposed an advanced process in which anodization is combined with chemical conversion, but such combined process is rather complicated and economically infeasible.

Whereas, it is an object of the present invention to provide an novel process for producing a protective, corrosionresistant film on an aluminum surface, which process will eliminate the foregoing difficulties of the various prior art processes.

It is a more specific object of the invention to provide a novel process which is capable of producing on an aluminum substrate an anodized under-coat and a resinous over-coat simultaneously in a single bath operation.

We have discovered that a desired corrosionresistant oxide film can be produced'on an aluminum or aluminum alloy surface by means of anodization employing an electrolyte which contains at least one of the compounds selected from the group consisting of aliphatic primary amines and aliphatic secondary amines. We have also ascertained that the oxide film greatly enhances adhesion of the coating composition to the aluminum surface. The oxide film according to the invention is substantially transparent, hence a variety of colors may be chosen for an over-coat to be applied to the aluminum product. U

In accordance with one aspect of the invention, there is provided a process which essentially comprises: de-

greasing, cleaning and water-rinsing the surface; etching the waterwashed surface with an alkaline solution; neutralizing and water-washing the etched surface; and anodizing the surface in an electrolyte containing 1.0-15.0 weight percent of at least one member selected from the group consisting of aliphatic primary amines and aliphatic secondary amines which is soluble under the experimental conditions in the aqueous solution. Another aspect of the invention resides in the provision of a process which comprises:' degreasing, cleaning and water-rinsing the surface; etching the waterrinsed surface with an alkaline solution; neutralizing and water-washing the etched surface; and anodizing the surface in an electrolyte containing 1.0-15.0 weight percent of at least one member selected from the group consisting of aliphatic primary amines and aliphatic secondary amines which is soluble under the experimental conditions in the aqueous solution and 20.0-50.0 weight percent of a water-soluble thermosetting resin; allowing the thus anodized surface to set; and drying and hardening the surface at a temperature of l50200c over a period of 15-30 minutes.

The electrolyte may' also contain an ETHODUOMEEN which is the trade name for ethylene oxide adducts with a fatty acid diamine: The term aliphatic primary amines includes ehtylenediamine and the like. The term aliphatic secondary amines includes dimethyl amine, N-methyl ethanolamine and the like. These compounds named as above have now been found capable of forming an oxide film on an aluminum surface when used in determined amounts. Amounts of less than 1.0 percent will result in poor oxide film and fail to enhance adhesion of the coating composition to the aluminum surface. Amounts in excess of 15.0 percent are too great to become soluble.

The term water-soluble thermosetting resins as used herein includes acrylic resins, alkyd resins, acrylalkyd resins, urea resins, aminoalkyd resins, melamine resins and phenolic resins. These resins mentioned as above when used in amounts of less than 20.0 percent or more 50.0 percent will fail to form a satisfactory resinous film on an aluminum surface.

The aluminum surface in the above second mentioned process is advantageously anodized for 5-30 minutes with DC 40-60 volts and a current density of 0.01-1 A/dm at a bath temperature of 50-90C. More advantageously, the surface can be anodized initially with a relatively low voltage i.e. DC 10-20 volts for 5-15 minutes, thereby forming thereon an oxide film and then with a relatively high voltage i.e. DC 30-60 volts for l-15 minutes thereby electrically depositing thereon a resinous film.

The following examples are provided to further illustrate the process of the invention, but these are not to be regarded as limiting.

EXAMPLE I An aluminum workpiece was subjected to the pretreatment in which it was degreased and cleaned by dipping for 30 seconds in 8 percent NaOH solution maintained at C, washed with water, neutralized by dipping for 15 seconds in a 15 percent I-INO solution at room temperature, and washed again with water. The pretreated substrate was subjected to anodization using an electrolyte containing 3 weight percent of ETHODUOMEEN (trade name for ethylene oxide adduct of fatty acid diamine) operated with DC 55 volts Table I Coating compositions (parts by weight) Adhesion (kg/cm") Coat Ethylene No. Resin (solid 100%) Butanol lsoproglycol Water Anionic Organic Compara- Example Example panol monosurfaceamine tive l 11 buthyl tant example ether 1 Water-soluble 22 9O 30 0.5 7.5 24.0 26.2 27.0

acrylic resin 100 2 Water-soluble 12 20 80 30 0.5 7.5 17.0 18.5 19.2

alkyd resin 100 3 Water-soluble acrylalkyd resin 100 12 20 8O 30 0.5 7.5 17.5 20.2 21.5

EXAMPLE VI source and a current density of 1 A/dm at a bath temperature of about 30c. The substrate was thus anodized for 15 minutes, whereupon there was obtained a transparent oxide film deposited on the aluminum workpiece.

EXAMPLE II An aluminum-bearing substrate was pretreated in the manner described in EXAMPLE I. The pretreated substrate was subjected to anodization using an electrolyte containing 10 weight percent of ethylenediamine operated with DC 30 volts and a current dinsity of 0.5 A/dm at a bath temperature of about 18C. The substrate was thus anodized for 30 minutes, whereupon a transparent oxide film was produced on the aluminum workpiece.

EXAMPLE III-V Aluminum workpieces were subjected to the pretreatment in which they were degreased and cleaned by dipping for 30 seconds in 8 percent NaOH solution maintained at 80C, washed with water, neutralized by dipping for 20 seconds in a percent NI-IO solution at room temperature, and washed again with water.

The thus pretreated substrates were subjected to anodization using electrolytes containing 1.0, 3.0 and 5.0 weight percent respectively of N-methyl ethanolamine operated with DC 50-100 volts and a current density of 0.15-0.55 A/dm at a bath temperature of about 11C. The substrate was thus anodized for 15 minutes whereupon a transparent oxide film was produced on each aluminum substrate.

COMPARATIVE EXAMPLE An aluminum substrate was pretreated in the manner described in EXAMPLE I. The pretreated substrate was subjected to anodization using a 15 percent H 80 electrolyte operated with DC 15 volts and a current density of l A/dm at a bath temperature of about C. The substrate was thus anodized for 15 minutes, then washed and dried thereby providing a comparative test piece. This test piece together with the alumi num piece obtained according to EXAMPLE I and EX- AMPLE II were immersed for 1 minute in an immersion bath containing the coating compositions shown in Table 1. They were taken out of the bath, disposed to set over a period of 10 minutes and thereafter were heated in an electrical furnace at a temperature of 180 for 20 minutes, until they were dried and hardened. The finished aluminum pieces were subjected to an adhesion test, with the results shown in Table 1. It will be seen that aluminum pieces obtained in the inventive examples were superior to one in a comparative example in respect of adhesion.

Table II Electrolyte Composition (weight percent) Titanium oxide Acrylic resin Melamine resin ETHODUOMEEN Glycol Ethylalcohol Water Surfactant every-Pa OOOOOONN EXAMPLE VII An aluminum substrate was pretreated in the manner described in EXAMPLE I. The pretreated substrate was subjected to anodization using an electrolyte of the composition shown in Table III operated with DC 40 volts and a current density of 0.5 A/dm at a bath temperature of C. The substrate was thus anodized for 15 minutes, and then pulled out, drained for 8 minutes, allowed to set for 7 minutes and thereafter heated at C for 30 minutes to harden the coating, whereupon there was produced a continuous, uniform, firmly adhered film on the aluminum workpiece.

Table III Electrolyte Composition (weight percent) Titanium oxide 370 Acrylic resin 15.0 Melamine resin 4.0 Ethylenediamine 10.0 Glycol 9.0 Ethylalcohol 1 6.0 Water 5.0 Surfactant 4.0

EXAMPLE VIII An aluminum substrate was pretreated in the manner described in EXAMPLES III-V. The pretreated sub- Table IV Electrolyte composition (weight percent) Acrylic resin 2 Melamine resin N-methyl ethanolamine Glycol Ethylalcohol Water Surfactant What is claimed is:

1. A process for forming a coating on an aluminum or aluminum alloy surface which comprises: degreasing, cleaning and water-rinsing the surface; etching the water-rinsed surface with an alkaline solution; neutralizing and water-washing the etched surface; and treating the surface as the anode to form an aluminum oxide layer thereon in an aqueous electrolyte containing 1.0 15.0 weight percent of at least one compound which is soluble in the aqueous solution under the experimental conditions selected from the group consisting of, aliphatic primary amines and aliphatic secondary amines; immersing the treated surface in a coating composition comprising 20-50 percent of at least one water-soluble thermosetting resin and 01-10 percent of at least one compound soluble in the aqueous solution under the experimental conditions, which compound is selected from the group consisting of, aliphatic primary amines and aliphatic secondary amines and allowing the thus coated aluminum surface to set, dry and harden at 200C. over a period of 15-30 minutes. 

1. A PROCESS FOR FORMING A COATING ON AN ALUMINUM OR ALUMINUM ALLOY SURFACE WHICH COMPRISES: DEGREASING, CLEANING AND WATER-RINSING THE SURFACE, ETCHING THE WATER-RINSED SURFACE WITH AN ALKALINE SOLUTION, NEUTRALIZING AND WATER-WASHING THE ETCHED SURFACE, AND TREATING THE SURFACE AS THE ANODE TO FORM AN ALUMINUM OXIDE LAYER THEREON IN AN AQUEOUS ELECTROLYTE CONTAINING 1.0-15.0 WEIGHT PERCENT OF AT LEAST ONE COMPOUND WHICH IS SOLUBLE IN THE AQUEOUS SOLUTION UNDER THE EXPERIMENTAL CONDITIONS SELECTED FROM THE GROUP CONSISTING OF, ALIPHATIC PRIMARY AMINES ANDALIPHATIC SECONDARY AMINES, IMMERSING THE TREATED SURFACE IN A COATING COMPOSITION COMPRISING 20-50 PERCENT OF AT LEAST ONE WATER-SOLUBLE THERMOSETTING RESIN AND 0.1-10 PERCENT OF AT LEAST ONE COMPOUND SOLUBLE IN THE AQUEOUS SOLUTION UNDER THE EXPERIMENTAL CONDITIONS, WHICH COMPOUND IS SELECTED FROM THE GROUP CONSISTING OF, ALIPHATIC PRIMARY AMINES AND ALIPHATIC SECONDARY AMINES AND ALLOWING THE THUS COATED ALUMINUM SURFACE TO SET, DRY AND HARDEN AT 150*-200*C. OVER A PERIOD OF 15-30 MINUTES. 